1. Field of the Invention
The present invention relates to aqueous polyurethane dispersion binders and processes for their production. The binders may be used, for example, in basecoatings, aqueous coatings, adhesives and printing inks.
2. Description of Related Art
Coating systems based on aqueous polyurethane dispersions have grown in importance over the last 15 years across a fairly broad range of applications because of their beneficial properties. These properties include adhesion to a variety of substrates, abrasion resistance, flexibility and toughness. Polyurethane dispersions are particularly suitable for the coating of metallic and mineral substrates and for coating plastics and wood. Polyurethane resins are generally stabilized in the aqueous phase by means of external emulsifiers, or by the incorporation of adequate quantities of ionic or nonionic groups into the polyurethane resin. The pattern of properties of polyurethane dispersions can be modified by, for example, adding vinyl polymer dispersions. U.S. Pat. No. 3,862,074, DE 39 15 459 and EP 0 379 158, selected by way of example, describe aqueous coating systems which can be prepared simply by mixing a polyurethane dispersion with acrylate dispersions.
Specific processes for preparing vinyl polymers, in which vinyl monomers are polymerized in the presence of aqueous polyurethane dispersions, are described in, for example, the patent documents EP 0 098 752, EP 0 167 188, EP 0 189 945, EP 0 308 115, EP 0 522 419 and EP 0 522 420. In the case of patent documents EP 0 189 945 and EP 0 308 115, the mixture of polyurethanes and vinyl polymers is prepared via the stage of a water-dispersible, isocyanate-terminated polyurethane resin which is synthesized in a solution of vinyl monomers and undergoes chain extension after transfer to the aqueous phase. This is followed by a free-radical polymerization in which further vinyl monomers can be added. In contrast, the patent document EP 0 167 188 discloses a process permitting the preparation of polyurethane-acrylate hybrid dispersions via the intermediate stage of an isocyanate-terminated polyurethane resin with terminal acryloyl groups. After dilution with compounds capable of undergoing free-radical polymerization, this unsaturated prepolymer is dispersed in the aqueous phase and subjected to chain extension. This is followed by a free radical-initiated polymerization in-the aqueous phase, to give a one-component dispersion having chemically bonded polyurethane and acrylate blocks.
EP 0 098 752, EP 0 522 419 and EP 0 522 420 likewise describe processes for preparing polyurethane-acrylic hybrid dispersions. They differ from the process disclosed in patent document EP 0 167 188 in that the synthesis of the polyurethane-acrylic hybrid dispersions is carried out via the stage of polyurethane macromonomers having terminal or lateral .alpha.,.beta.-olefinically unsaturated groups, which do not possess terminal isocyanate groups and therefore cannot be chain-extended in the aqueous phase. Here too, after transfer of these polyurethane macromonomers to the aqueous phase, a free radical-initiated polymerization is carried out in the presence of mono- and/or polyfunctional vinyl monomers. None of the above mentioned polyurethane-vinyl hybrid dispersions or mixtures of polyurethane and vinyl-polymer dispersions is autocrosslinked at room temperature or at low temperatures.
Vinyl polymers containing carbonyl groups were first mentioned in the patent literature more than 20 years ago. They are generally prepared by an emulsion polymerization process, by polymerization of carbonyl-containing vinyl monomers with other vinyl monomers. Examples of such processes are given in patent documents DE 15 95 393, DE 28 19 092, EP 0 127 834 and EP 0 332 011.
Autocrosslinking aqueous coating systems based on mixtures of polyurethane resins and vinyl polymers are disclosed in patent document EP 0 332 326. In this patent autocrosslinking is achieved via azomethine linkages which are the result of a reaction between hydrazine and carbonyl groups. Crosslinking in these coating compositions involves the participation of at least one polyurethane resin which has hydrazine or carbonyl groups in the polymer backbone.
A preferred variant preparation of the polyurethane polymer disclosed in EP 0 332 326 is its synthesis in vinyl monomers which, after the dispersion of the polyurethane resin in aqueous medium, are subjected to free-radical polymerization. This process leads to a polymer mixture comprising polyurethane resin and vinyl polymer.
The patent document also mentions mixtures of polyurethane polymers and vinyl polymers, both of which carry carbonyl groups and can be crosslinked by polyhydrazides. Where a carbonyl function is involved, it is incorporated into the polyurethane resin during the synthesis of the prepolymer and/or during the chain extension process. Both possibilities require isocyanate-reactive compounds having carbonyl functions. The crosslinking of this carbonyl-containing polyurethane resin can be carried out using hydrazine-containing polyurethane polymers and vinyl polymers, and with polyhydrazides which are not of the polyurethane or vinyl type. In this case the urethane polymers and vinyl polymers are present as a purely physical mixture. It is known that such systems have a tendency to separate, resulting not only in an inadequate storage stability but also in an impairment of the mechanical properties, the resistance properties and the appearance of the film (e.g., defects in gloss, hazy films).